The present invention relates to a power feed apparatus used in high-frequency heater, which is adapted to convert power supplied through a transformer and provided from a power supply such as commercial supply or the like so as to feed the power to a load having the reverse blocking characteristics of a magnetron or the like.
Generally a magnetron power supply apparatus is an apparatus which converts power supplied by a transformer or the like to and to feed the power to, for example, a magnetron or a load having such reverse blocking characteristics.
FIG. 1 is a circuit diagram of one conventional magnetron power supply apparatus. In the drawing, the output of the power supply E provided by a commercial power supply 1 or the like is stepped up by a step up transformer T, and rectified by a capacitor C and a diode D and fed to a magnetron M. Accordingly, the waveform V.sub.AK of the voltage fed to the magnetron M which is a load having the reverse blocking characteristics is shown in FIG. 2(b) with respect to the power supply voltage waveform shown in FIG. 2(a). The input current I from the power supply E continuously flows as shown in FIG. 2(c). The transformer T and the power supply E stably operate without causing any inconvenience even if the load has the reverse blocking characteristics.
However, the diode D is high in cost because of the requirement of the high voltage withstanding capability and further is easily destroyed by an excessive surge current or the like through the magnetron M, so that a power feed system, which is free from this disadvantage, is desired. Since rectification at a high-frequency is required to be performed at a high voltage when the power supply E is a high-frequency power supply, it is difficult to make a diode of sufficient capability and the price can become extremely high even if it is made.
When a power feed apparatus of a type where the capacitor C and diode D of FIG. 1 are omitted may be realized as in FIG. 3, such inconveniences as described hereinabove are caused so as to make it difficult to realize the apparatus.
Namely, in the case of the apparatus of FIG. 1, the track on the B-H curve of the transformer T is 0.fwdarw.a.fwdarw.b.fwdarw.c.fwdarw.d.fwdarw.a, and in the case of an apparatus of FIG. 3, it is 0.fwdarw.a.fwdarw.b.fwdarw.c as shown in FIG. 4, thus causing the so-called deviated magnetism phenomenon, with the result that the transformer becomes very inferior in its efficiency, and the influences are worse on the operation of the power supply E.
From this background, a prior art high-frequency inverter power supply of a fly-back type for the power supply E is proposed as in FIG. 5. FIG. 5 is a circuit diagram of the other conventional power feed apparatus, and is power-supply circuit diagram of a high-frequency heating apparatus described in U.S. Pat. No. 4,318,165.
In FIG. 5, the power of the commercial power supply is rectified by a diode bridge 2, thus forming a fullwave rectified DC power supply. An inductor 3 and a capacitor 4 form a filter with respect to the high-frequency switching operation of an inverter.
The inverter is composed of a resonance capacitor 5, a step-up transformer 6, a transistor 7, a diode 8 and a driving circuit 9. The transistor 7 is caused to switch with a given period and duty cycle (namely, on to off time ratio) by the base current supplied from the driving circuit 9. As a result, a current Icd, with a collector current Ic and a, diode current Id as shown in FIG. 6(a) flows to the primary winding 10 of the transformer 6, and a high-frequency current I.sub.L as shown in FIG. 6(b) flows through the primary winding 10. Thus, a high-frequency high-voltage and a high-frequency low-voltage are respectively generated across the second winding 11 and the third winding 12 of the transformer 6. The high-frequency low-voltage is fed between the cathode terminals of the magnetron 17 through the capacitors 13 and 14 and the choke coils 15 and 16 while the high-frequency high-voltage is fed as shown between the anode and cathode of the magnetron 17. Currents as shown in FIGS. 6(c) and 6(d) respectively flow through the capacitor 5 and the magnetron 17 so that the magnetron 17 oscillates to make dielectric heating possible.
Such a construction as described hereinabove has characteristics such that the weight and size of the step-up transformer may be considerably reduced, as compared with using a step-up transformer at the commercial power-supply frequency, and, when the transistor 7 is operated with a frequency in the range of approximately 20 kHz through 100 kHz, the power-supply portion may be made smaller in size and, lower in cost.
Particularly, the high-frequency heating apparatus shown in U.S. Pat. No. 4,318,165 is constructed in the so-called fly-back type converter circuit in which the polarities of the primary winding 10 and the secondary winding 11 of the transformer 6 are shown, so that the magnetron can be driven without the use of the high-voltage diode normally used for the high-voltage rectification, thus realizing such a high-frequency as shown in FIG. 5.
Accordingly, since the high-voltage high-frequency diode which is extremely high in price and larger in size becomes unnecessary, and the higher-frequency heating apparatus is smaller, lighter, and less expensive.
However, such a conventional high-frequency heating apparatus as described hereinabove has the following defects. A converter or an inverter which is a type of power converter is described in detail in, for example, a document by L. E. Jansson "Converter Circuits for Switched-mode Power Supplies" Electronics Applications Bulletin, Vol. 32, No. 3, N. V. Philips (1973). There are a fly-back system and a forward system as a converter using one transistor. It is known that the fly-back system of converter is often used in the high-voltage producing circuit for television use, because it has the least number of components, and may be constructed lower at price.
However, in the case of handling a large amount of power as in the energy appliance, the characteristics are considerably reduced. At page 86 through page 87 of the document, this is described in detail, the addition of the various components is required to provide an output of, for example, approximately 200 W or more, to realize a converter, for handling a large amount of power of 200 W or more, by the fly-back system, the components become complicated and the price becomes higher. Also, although it is ideal to construct the leakage inductance of the transformer so as to be zero, especially for the fly-back converter, it is really difficult to realize this, so that important influences are applied to the semiconductor switch element such as transistor or the like. Since this influence becomes important as the power handled by the converter becomes larger, a protective apparatus which is bothersome and large in size is required to protect the transistor from the influence. It is therefore not suitable to apply the converter of the fly-back system for the high-frequency heating apparatus handling high power (for example, approximately 1 through 2 kW).
On the other hand, when the output of the transformer 6 is connected directly to the magnetron 17, the polarity of the transformer is one of the converter of the forward system as shown in FIG. 7, such inconveniences as described in with respect to the system shown in FIG. 3 are caused, that is, the so-called deviated magnetism phenomenon is caused to make the operation of the converter unstable.
Namely, as shown in FIG. 4, the operation track on the B-H curve of the voltage transformer 6 does not become the operation track of the normal transformer of the 0.fwdarw.a.fwdarw.b.fwdarw.c.fwdarw.d.fwdarw.a, but the track of the 0.fwdarw.a.fwdarw.b.fwdarw.a, where the efficiency becomes very inferior and the deviated magnetism phenomenon is likely to be caused. Accordingly, in the construction where the diode is omitted, it is extremely difficult to feed power to a magnetron having the reverse blocking characteristics using the forward system converter.
Furthermore, the anode current 1A of the magnetron 17 becomes the current waveform having a large peak value as shown in FIG. 6(d). This is because of the fact that in the so-called fly-back type converter, the energies accumulated in the primary winding 10 for the period of time that the transistor 7 is conductive are discharged to the magnetron 17 through the secondary winding 11 for a non-conductive period. Also, since the current flows to the magnetron 17 only during the non-conductive period of the transistor 7, the peak value of the anode current I.sub.A must become much larger to provide a given average current for getting a given radio wave output.
Thus, the emission capability of the cathode of the magnetron 17 has to be enlarged, so that the magnetron 17 becomes higher in price. Also, when the finishing peak value of the anode current I.sub.A is large, the abnormal oscillation phenomenon, that is, the so-called moding phenomenon, in the frequencies except for the given frequency with respect to the emission capacity surplus is likely to be produced, thus considerably shortening the service life of the magnetron; also, since such frequencies exclude the given frequency, there are inconveniences in that the wave leakage amount of the high-frequency heating apparatus increases so as to limit the lower price of the high-frequency heating apparatus or to lower the reliability.